Ivyleaf morningglory weed-free period, weedy period, and density studies were conducted in 1993 and 1994 to determine the effects of soil-applied imazaquin and chlorimuron plus metribuzin on ivyleaf morningglory interference in soybean. A weed-free period of 4 wk was needed to maintain soybean yield equivalent to weed-free season long when herbicides were not used, but treatment with either chlorimuron plus metribuzin or imazaquin maintained yield comparable to weed-free at all intervals. Ivyleaf morningglory biomass was reduced by both herbicides across all weed-free periods. The period nontreated ivyleaf morningglory plants were tolerated without yield loss was only 2 wk, but this interval was extended to 8 wk after treatment with chlorimuron plus metribuzin or imazaquin. Without herbicides, soybean yield was reduced by four ivyleaf morningglory per m of row, but the highest population evaluated in this study (eight per m of row) did not reduce soybean yield after treatment with chlorimuron plus metribuzin or imazaquin. Ivyleaf morningglory biomass was reduced at all densities after treatment with either herbicide.

Three experiments evaluated sicklepod interference with soybean with and without preplant incorporated applications of chlorimuron plus metribuzin or imazaquin. Sicklepod density, weed-free period, and weedy period were examined. In the absence of herbicides, soybean seed yield was reduced with 2 sicklepod plants row m-1, whereas 8 plants row m-1 were necessary to reduce yield when herbicides were used. Herbicide use also increased soybean yield at higher sicklepod densities. Chlorimuron plus metribuzin reduced sicklepod dry matter at 8 plants row m-1. To maintain soybean yield, a weed-free period of 4 wk after emergence was required, regardless of treatment. Both herbicide treatments resulted in increased soybean yield at the zero and two wk weed-free periods; however, they did not affect soybean yield when the weed-free period was 4 wk or more. Imazaquin reduced sicklepod density when plots were left weedy full-season, and further reductions were noted with chlorimuron plus metribuzin. A sicklepod weedy interval of 8 wk reduced soybean yield when untreated, but either herbicide treatment extended that interval to 16 wk.

Sicklepod seed production and germination were evaluated in density and weed-free period studies without herbicide treatment and after imazaquin or metribuzin plus chlorimuron were applied PRE in soybean. Neither herbicide nor sicklepod density affected sicklepod seed germination, but delaying emergence of sicklepod from 0 wk to 8 wk after soybean emergence reduced subsequent sicklepod seed germination from 92% to 48%. Increasing sicklepod density from two to eight plants per row m reduced sicklepod seeds per plant, but increased sicklepod seeds per m2, regardless of herbicide. Extending the weed-free period from 0 to 8 wk reduced sicklepod seeds per plant and seeds per m2. Imazaquin and chlorimuron plus metribuzin reduced sicklepod seeds per m2 more than 50% at 0 or 2 wk weed-free intervals, compared with nontreated plants at these same intervals.

Studies were conducted to determine the effect of in-row eastern black nightshade establishment and removal timings in plasticulture tomato on tomato yield loss and nightshade berry production and seed viability. Eastern black nightshade was transplanted at 1, 2, 3, 4, 5, 6, and 12 wk after tomato planting (WAP) and remained until tomato harvest, or was established at tomato planting and removed at 2, 3, 4, 5, 6, 8, and 12 WAP to determine the critical weed-free periods. Eastern black nightshade seed viability increased with berry size and with length of establishment or removal time. The critical weed-free period to avoid viable nightshade seed production was 3–6 WAP. Tomato yield decreased with early weed establishment or with delayed time of weed removal. The critical weed-free period to avoid greater than 20% tomato yield loss for the sum weight of extra large and jumbo grades was 28 to 50 d after tomato transplanting.

Crop–weed interactions is an important topic for introductory weed science courses. The effect of the timing of weed emergence and the duration of weed competition on crop yield are two topics usually covered when discussing competition. Students generally gain a better knowledge of these concepts through observation in addition to discussion of the underlying concepts. An additive removal and plant-back experiment was used, in the undergraduate weed science laboratory at Iowa State University, to demonstrate critical period for weed control concepts for cultivated radish. In one series of treatments, weeds were sown at the time of radish planting and removed at 2, 3, and 4 wk after planting. In another series, weeds were sown at 1, 2, and 4 wk after radish planting and allowed to grow in the flats until completion of the experiment. Two controls, one weed free and one unweeded, were also included. The results from four semesters suggested that the critical period for weed control began immediately after planting and lasted 3 wk. The timing and duration of the critical period was consistent across the four semesters evaluated. This activity was successful in demonstrating the critical period for weed control in radish.

Field research trials were conducted in Idaho at the Aberdeen Research and Extension Center in 2006 and 2007 to determine the critical interference period of hairy nightshade in potatoes. ‘Russet Norkotah’ variety was planted both years in plots three rows wide by 12 m long. When the potatoes had emerged, one- to two-leaf hairy nightshade plants that had been germinated and grown in the greenhouse were transplanted at a 2 m−1 row density and allowed to grow for 10, 20, 30, or 40 d after emergence (DAE) before removal, or the potatoes were maintained weed-free for 0, 10, 20, 30, or 40 DAE before transplanting. Potatoes were harvested from the center row at the end of each growing season and yield and grade was determined. Russet Norkotah is a small-canopied potato variety and often does not completely close canopy in Idaho. Russet Norkotah U.S. No. 1 and total tuber yield were similar to weed-free yield when hairy nightshade transplanting was delayed up to 22 or 24 DAE, respectively, or planted at emergence and allowed to remain for only 6 or 11 d, respectively. Otherwise, yield decrease was 5% or greater. Therefore, the critical weed-free period for Russet Norkotah potato U.S. No. 1 or total tuber yields was 6 to 22 or 11 to 24 DAE, respectively.

Weeds are often the major biological constraint to growing legume crops successfully, and an understanding of the critical period of weed control (CPWC) is important for developing environmentally sustainable weed management practices to prevent unacceptable yield loss. Therefore, we carried out two field experiments to identify the CPWC for two grain legume crops traditionally grown in Mediterranean areas: chickpea and faba bean. The experiments were conducted at two sites both located in the Sicilian inland (Italy). In chickpea, when weeds were left to compete with the crop for the whole cycle, the grain yield reduction was on average about 85% of the weed-free yield, whereas in faba bean the reduction was less severe (on average about 60% of the weed-free yield). The onset of the CPWC at a 5% yield loss level varied by species, occurring later in faba bean than in chickpea (on average, 261 and 428 growing degree days after emergence for chickpea and faba bean, respectively). In both species, the end of the CPWC occurred at the early full-flowering stage when the canopy of each crop enclosed the interrow space. On the whole, the CPWC at a 5% yield loss level ranged from 50 to 69 d in chickpea and from 28 to 33 d in faba bean. The results highlight the fact that faba bean has a higher competitive ability against weeds than chickpea. This could be attributable both to more vigorous early growth and to the plant's greater height, both factors related to a greater shading ability and, consequently, to a better ability to suppress weeds.

Field studies were conducted in 2005 at five locations throughout Illinois to determine the effect of maturity date and canopy width characteristics of selected glyphosate-resistant soybean cultivars on competitive ability against weeds. Natural weed populations at all sites were allowed to compete with the soybean cultivars until the V1, V3, V5, or V7 growth stage and then removed with an application of glyphosate. Light-interception readings, weed control ratings, and soybean yield were all measured to compare cultivars. Cultivars of later maturities tended to withstand early season weed competition better and attained higher yields when weeds were removed at later timings than those of earlier maturity. Although canopy width differed moderately among cultivars in some cases, canopy width had no effect on the ability of soybeans to compete with weeds. Thus, choosing soybean cultivars of later maturity might provide more flexibility in weed management and might reduce losses due to weeds, but that benefit needs to be balanced with yield potential.

Field studies were conducted in 1996, 1997, and 1998 at Clinton, NC, to determine the influence of Palmer amaranth establishment and removal periods on the yield and quality of plasticulture-grown ‘Mountain Spring' fresh market tomato. Treatments consisted of 14 Palmer amaranth establishment and removal periods. Half of the treatments were weed removal treatments (REM), in which Palmer amaranth was sowed at the time tomato transplanting and allowed to remain in the field for 0 (weed-free all season), 2, 3, 4, 6, 8, or 10 wk after transplanting (WAT). The second set of the treatments, weed establishment treatments (EST), consisted of sowing Palmer amaranth 0 (weedy all season), 2, 3, 4, 6, 8, or 10 WAT and allowing it to grow in competition with tomato the remainder of the season. Tomato shoot dry weight was reduced 23, 7, and 11 g plant−1 for each week Palmer amaranth removal was delayed from 0 to 10 WAT in 1996, 1997, and 1998, respectively. Marketable tomato yield ranged from 87,000 to 41,000 kg ha−1 for REM of 0 to 10 WAT and 28,000 to 88,000 kg ha−1 for EST of 0 to 6 WAT. Percentage of jumbo, large, medium, and cull tomato yields ranged from 49 to 33%, 22 to 31%, 2 to 6%, and 9 to 11%, respectively, for REM of 0 to 10 WAT and 30 to 49%, 38 to 22%, 3 to 2%, and 12 to 9%, respectively, for EST of 0 to 6 WAT. To avoid losses of marketable tomato yield and percentage of jumbo tomato fruit yield, tomato plots must remain free of Palmer amaranth between 3 and 6 WAT. Observed reduction in marketable tomato yield was likely due to competition for light as Palmer amaranth plants exceeded the tomato plant canopy 6 WAT and remained taller than tomato plants for the remainder of the growing season.